The regional distribution of ventilation and blood flow in the lung will be measured using the radioactive xenon method. The effect on regional ventilation and perfusion in asthmatic patients will be measured before and after inhalation of the bronchoconstrictor, methacholine, or of an aerosolized antigen in order to determine whether mismatching of gas and blood flow in large regions of the lungs contributes to the hypoxemia which commonly occurs during an asthmatic attack. In patients with progressive systemic sclerosis (PSS) the regional distribution of lung blood flow will be measured during breathing of a hypoxic gas mixture and after Raynaud's phenomenon is induced by chilling. The purpose of these experiments is to see whether pulmonary vascular responses (assessed indirectly from the change in blood flow distribution) are abnormally increased or blunted in PSS, a possible contributing factor to the lung fibrosis which occurs in that condition. In normal subjects regional lung blood flow will be measured at varying low lung volumes near the residual volume and the results will be correlated with the "closing volume". The purpose of these studies is to see whether a phenomenon of "small vessel closure" analogous with "small airways closure" can be identified in the normal lung.